Conformal Dimensions of Two-Derivative BMN Operators

Transcription

1 Preprint typeset in JHEP style - HYPER VERSION hep-th/ AEI Conformal Dimensions of Two-Derivative BMN Operators Thomas Klose Max-Planck-Institut für Gravitationsphysik Albert-Einstein-Institut Am Mühlenberg 1, D Golm, Germany thomas.klose@aei.mpg.de Abstract: We compute the anomalous dimensions of BMN operators with two covariant derivative impurities at the planar level up to first order in the effective coupling λ.the result equals those for two scalar impurities as well as for mixed scalar and vector impurities given in the literature. Though the results are the same, the computation is very different from the scalar case. This is basically due to the existence of a non-vanishing overlap between the derivative impurity and the background field Z. We present details of these differences and their consequences. Keywords: AdS-CFT Correspondence, Penrose limit and pp-wave background, 1/N Expansion.

2 Contents 1. Introduction The Berenstein-Maldacena-Nastase (BMN) correspondence [1] is a limit of the AdS/CFT duality [2] between type IIB superstring theory on AdS 5 S 5 and N = 4 supersymmetric Yang-Mills theory on R 4. The novel feature is that there exists a regime of the effective coupling strengths where both theories can be treated perturbatively at the same time. On the string theory side the BMN limit amounts to a Penrose limit [3] of AdS 5 S 5 and leads to a plane-wave geometry [4] which is labeled by a parameter µ of unit mass dimension. It is the geometry which a particle experiences that travels with large angular momentum J along a circle S 1 on the 5-sphere S 5. Choosing one particular circle corresponds in the super Yang-Mills theory to singling out a U(1) subgroup of the SU(4) R-symmetry group. In consequence, the string states are represented by operators with large U(1) R charge J which are commonly named BMN operators. The correspondence is made more precise by the following dictionary. The string theory Hamiltonian Ĥl.c. in light-cone gauge is identified with the Yang-Mills dilatation operator ˆD and the generator Ĵ of U(1) R transformations by the relation The relationship (??) has been written as an operator equation. However, both sides act on very different Hilbert spaces. Therefore it is necessary to know how string states are translated into BMN operators and vice versa. This question is a subject of current research [8] [12]. Since the original proposal of BMN a lot of work has been done to understand the correspondence in more detail especially at the interacting level. Some further references for investigations on the gauge theory side are [13] [24] and for the string theory side see e. g. [25] [41]. In [42][43][44] a string bit formalism has been developed which interpolates between the two sides. In the following we summarize the mapping between string states and BMN operators at the planar level (g 2 = 0) where the dictionary is well established. In this case the Hamiltonian simplifies to that of a non-interacting massive string which can be solved exactly [25][26]. In terms of occupation numbers N k the Hamiltonian reads In the diagonal basis underlying (??) the relation(??) becomes 1

3 The (single) string vacuum 0,p + has zero energy and therefore should be represented by an operator whose conformal dimension equals its R-charge. The operator with this property, proposed by BMN [1], is The excited states Eq. (??) has been verified for operators with scalar field impurities to second order in λ in [13] and to all orders in [17], and for mixed scalar field and covariant derivative impurities up to O (λ ) in [19]. In this paper we confirm (??) to first order in λ for operators with two covariant derivative impurities. On the string theory side (at least at the non-interacting level) there is no difference between the oscillation modes in the directions µ =1, 2, 3, 4 and i =5, 6, 7, 8. However the gauge theory calculations with derivative impurities are very different from the case of scalar impurities, in fact they are much more complex. This is ultimately due to the overlap between derivative impurity and background field Z which vanishes for scalar impurities. Furthermore one has to cope with more diagrams stemming from insertions of the gauge field that is contained in the derivative impurity. Therefore it is very interesting to see how the result emerges in this more intricate case. The paper is organized as follows. In the next section we introduce the operators we want to study and discuss some of their properties. In section 3 we present in detail the computation of their one-loop anomalous dimensions in the planar BMN limit. We conclude by summarizing the specialties of this computation in section 4. Our notation and conventions can be found in the appendix. 2. Two-derivative BMN operators and their conformal dimensions In this work we are concerned with operators with two covariant derivative insertions D µ and D ν. These represent the string states (α µ n) (α ν n ) 0,p + with energy For the definition of the zero mode operators (n =0)wetake(??) with an additional normalization factor of J 1. This is natural since e 2πinp/J is of order O (J) forn =0 and of order O (1) otherwise. Apart from that, the definition (??) simplifies drastically in absence of the phase factor. The commutator terms cancel and the derivatives can be taken out of the trace: The set of operators (??),(??) transforms under a reducible tensor representation of SO(4). A decomposition into subsets that transform under (different) irreducible representations lead to the following linear combinations (n Z) D (µν),n := 1 ) 2( Dµν,n + D νµ,n 1 4 δ µνd κκ,n, (2.1a) D [µν],n := 1 ) 2( Dµν,n D νµ,n, (2.1b) D n := D κκ,n, (2.1c) 2

4 which represent the symmetric-traceless part, the anti-symmetric part and the trace, respectively. Our aim now is to compute the conformal dimensions n = (0) + δ n of these operators. The notation anticipates that the conformal dimensions will depend on the mode number but not on the SO(4) irrep. They can be read off from the two point correlator whose generic form is Of course it has to be checked that the correlators of the operators defined in (??) and (??) indeed have the required form (??). In fact, we will find that for the introduced operators this is not the case and we are compelled to redefine the non-zero mode operators (??) by adding a suitable proportion of the zero mode operators (??), cf. (??). At the technical level the one-loop anomalous dimension is determined as follows. Computing the two point correlator (??) at tree level fixes the overall constant C µν,ρσ. Afterwards one obtains the relative factor, which essentially is the anomalous dimension, from the one-loop computation. In the latter calculation we use dimensional reduction to d dimensions in order to regularize divergences and find the renormalized operators. In what follows we will perform this calculation. We consider the BMN limit (??) and furthermore restrict ourselves to diagrams that can be drawn on a sphere without crossing lines. As mentioned in the introduction, our result 3. The one-loop computation 3.1 Preliminary remarks The first thing to note is that the insertion of a covariant derivative actually produces two terms, one with partial derivative and one with the commutator of the gauge field A. To be more concrete let us split the operator (??) into pieces with no, one and two A-fields, respectively, as In order to figure out what happens at one-loop order, we need to discuss the possible vertices. First of all there are the same interactions as in the scalar case, namely the scalar self-energy, the four scalar interaction and the gluon exchange. All of these contribute to the next-to-leading order in g YM when inserted into the two point function of d (0) with itself. However, in our case there is also an additional contribution to this order stemming from the cross term of d (0) with d (1) which can be connected by the gluon emission vertex. Let us collect all relevant terms in the definition In the next subsections we will work out (??) and(??) in detail. For that purpose let us introduce some technicalities. For the zero mode operators the derivatives could be 3

5 pulled out of the trace, cf. (??). But also the non-zero mode operators (??) can be written with the derivatives taken in front by utilizing the idea of the q-derivative [19] There is another important property of (??). Although the fields Z have been moved to different coordinates the cyclicity of the expression is retained. Since q J = 1 the coordinates can be relabeled cyclicly without changing the phase factor. Explicitly we have for some arbitrary 1 function f(x 1,...,x J ) 3.2 Classical Correlator O ( g 0 ) YM In this section we evaluate the tree level correlator (??). For n, m 0 it reads in terms of (??) The evaluation of (??) goes as follows. At tree level the expectation value is given by J propagators that connect the two traces. The first connection can be chosen arbitrarily but the others are determined by planarity. However, using (??), we can always relabel the coordinates in such a way that Z(x 1 ) is connected to Z(y 1 )andz(x 2 )to Z(y 2 ) etc. This produces a factor of J and another factor N J comes from the fact that the web of propagators consists of J closed color lines. This leads to 3.3 Quantum Corrections O ( g 2 ) YM Now we turn to the evaluation of the quantum corrections to the D D-correlators (??), which are summarized in (??). To begin with we investigate only the case n, m 0;the other cases are treated in the summary on page??. Explicitly we can write (??) as The evaluation of this expression will simplify if we combine it with the other terms of (??). Therefore let us write d (1) µν,n explicitly: For the terms of the last two lines in (??) wecomputetheoverlapof(??) with itself, or more precisely with 1 The only restriction is that D x i µ f makes sense. 4

6 Now we know all relevant terms in (??) and list them in their pictorial version The fields Z(x 1 ), Z(x 2 ), Z(y1 ), and Z(y 2 ) are connected by some four point vertex which we will simply denote by After the derivatives have been applied all coordinates x k and y l are taken to coincide, respectively, which we depict by In the following we wish to show the evaluation of (??). We will successively consider the cases where none, one, two, three and all four derivatives act on the vertex. It will turn out that only the case with exactly two derivatives leads to non-vanishing contributions. If the number of derivatives is less than two, the vertices are zero and diagrams with more than two derivatives applied to the vertex do not contribute in the BMN limit Vertex without any derivative The bare vertex is given by (??) which becomes Vertex with one derivative Without loss of generality let us investigate the case where the derivative sits at the upper left leg Vertex with two derivatives This is the first case with non-zero vertices. As mentioned above it is the only case contributing to the anomalous dimension in the BMN limit. There are four diagrams with different placements of the two derivatives that we need to know: Vertex with three derivatives There are the following vertices Vertex with four derivatives In this last case, there are four essentially different vertices 5

7 3.3.6 Summary For the non-zero mode operators (n, m 0)wehavejustfound Let us now come to the cases where n =0orm = 0 or both. Assume first n 0,m =0 where the expression analogous to (??) reads Finally it remains to apply the transformation (??) in order to find the correlators of the redefined operators (n, m 0): D µν,n D ρσ,m 1 = λ n 2 L ε (w) δ n,mj µρ J νσ + δ n, m J µσ J νρ (w 2 ) J+2, (3.1a) D µν,n D ρσ,0 1 = λ n 2 4 δ 2w ρ w σ µν w 2, (3.1b) D µν,0 D ρσ,m 1 = λ m 2 4 δ 2w µ w ν ρσ w 2, (3.1c) D µν,0 D ρσ,0 =0. (3.1d) 1 In this way we again got rid of the unwanted terms in (??). However, we could not abolish the overlap between non-zero and zero mode operators, which is basically due to the fact that (??) vanishes and cannot aid in the transformation (??). It is therefore necessary to perform another redefinition. This time it will involve the coupling constant λ,since the plan is to make use of the tree level correlator (??) in order to remove the overlap at one-loop level. The suitable redefinition is (n 0) 3.4 Anomalous dimension We add the quantum corrections (??) to the classical part of the correlators (??): D µν,n D µν,0 δ n,m J µρ J νσ + δ n, m J µσ J νρ ( D ρσ,m = 1+λ (w 2 ) J+2 n 2 L ε (w) ), (3.2) 4w µ w ν w ρ w σ /w 4 = D ρσ,0 (w 2 ) J+2 (3.3) and without overlap between non-zero and zero modes. Recall that this result is only valid in the BMN limit and that it represents only the planar part of the correlator. In order to read off the anomalous dimension we actually have to renormalize the non-zero mode operators by 6

8 4. Conclusion Our aim was to find BMN operators that represent the plane-wave string states Actually this kind of operators had been studied before. In [22] N. Beisert derived operators representing the state (??) by arguments using superconformal symmetry. They look different for different SO(4) irreps. The exponential phase factors are replaced by sine and cosine functions. These operators are well-defined also outside the BMN limit for finite J. However for the limit (??) where the BMN correspondence is supposed to hold, we have shown that (??) represents(??), at least up to one-loop order and in the planar limit. A practical reason prefers (??) to the finite J BMN operators of [22]. If one wants to actually compute the anomalous dimensions from diagrams it proves advantageous to utilize the q-derivative of [19]. But then we need a phase factor q with q J =1inorderto retain the cyclicity of the trace and hence we are led to exponentials. As we have shown at length in section 3 the form of the operators allowed us to write the two point correlation function in a way convenient for the one-loop calculation, cf. (??). The details are rather intricate but effectively it amounts to a correlator of tr Z J and tr Z J with one generic vertex and four derivative operations acting onto the whole object. Working with this expression in the subsequent computations consisted in simple graphical manipulations following merely from Leibniz rule and reflection symmetries. The results were the following. If none or one derivative act onto the vertex, the vertex vanishes. If three or all four derivatives act onto the vertex, the diagrams are negligible in the BMN limit. Only the case where two derivatives act onto the vertex contributes to the anomalous dimension. Regrettably, eq. (??) could only be found so easily in the planar limit. Though one could find effective vertices in general, all the nice and valuable relations between them only hold in the planar case. Moreover, in our planar computation we could move all different interactions on top of each other (always occurring between Z(x 1 ), Z(x 2 ), Z(y1 ) and Z(y 1 )) and see them frequently cancel each other. This will not be possible in general and one has a large number of effective vertices. And as opposed to the case of scalar impurities, this cancellation is believed not to take place before the summation over the impurity insertion points. This is basically due to an essential difference between derivative and scalar case, namely the fact that derivative impurities have non-vanishing overlap with the background field Z, whereas scalar impurities do not: It is left for the future to find the subset of all vertices that are in the end relevant for computations involving two derivative BMN operators. This will hopefully be the analogue of the effective scalar vertex such that the symmetry between the two cases would be more apparent. 7

9 Acknowledgments My special thanks go to Jan Plefka and Niklas Beisert for introducing me to the topic, for valuable discussions and useful hints. For further helpful discussions I would like to thank Ari Pankiewicz, Matthias Staudacher, Charlotte Kristjansen and Markus Pössel. A. Appendix: Yang-Mills theory We use the following Euclidean action of N = 4 supersymmetric Yang-Mills theory in d =4 dimensions: In the BMN correspondence one singles out a U(1) = SO(2) subgroup of the SU(4) = SO(6) R-symmetry group. Label the scalar fields such that this U(1) subgroup rotates φ 5 and φ 6 into each other. Then Z := 1 2 (φ 5 + iφ 6 ) carries unit charge with respect to this U(1). The complex conjugated field Z possesses charge 1, whereas the remaining 4 scalar fields are neutral. The propagators and vertices are conveniently written in terms of the following functions which were defined in [20] d d p e ip(x1 x2) I 12 := (2π) d p 2 = 1 Γ( d 2 1) d 4 4π d/2 [(x 1 x 2 ) 2 ] d/ π 2 (x 1 x 2 ) 2, (A.1a) Y 123 := d d ui 1u I 2u I 3u, (A.1b) X 1234 := d d ui 1u I 2u I 3u I 4u, (A.1c) H 12,34 := ( 1 2 ) ( 3 4 ) d d ud d vi 1u I 2u I uv I v3 I v4. (A.1d) The arguments have been written as indices with the meaning 1 x 1 etc. The propagators for scalar fields and the gauge field are Z a (x) Z b (y) = g2 YM 2 tr T a T b I xy, (A.2) A a µ (x)a b ν (y) = g2 YM 2 δ µν tr T a T b I xy, (A.3) respectively. The required vertices are given in the main text. References [1] D. Berenstein, J. M. Maldacena and H. Nastase: Strings in flat space and pp waves from N = 4superYangMills,JHEP0204 (2002) 013, hep-th/ [2] O. Aharony, S. S. Gubser, J. M. Maldacena, H. Ooguri and Y. Oz: Large N field theories, string theory and gravity, Phys. Rept. 323 (2000) 183, hep-th/

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